1. Field of the Invention
This invention relates to movable stages and other precision location mechanisms and more specifically to an improved mirrored arrangement for a stage whose position is determined by an interferometer.
2. Description of the Prior Art
It is well known to use an interferometer arrangement to determine and control the position in the XY axes of a movable mechanical stage, for instance in high resolution microlithography applications. FIG. 1 shows in a plan view a stage 10 capable of moving in the X and Y axes. (The X and Y axes shown are not a structural element but merely for orientation purposes). Stage 10 is conventionally driven along both the Y and X axes. Typically this is accomplished by linear motors, a first set of linear motors providing the X direction motion and a second set of linear motors providing the Y direction motion. For instance, the stage powered by linear motors may be moved back and forth in the Y direction on a mechanical guide beam which in turn is moved and powered by linear motors in the X direction along guide rails. This provides the desired independent two direction motion.
Such a stage 10 supports for instance an integrated circuit wafer so that the wafer can be precisely positioned for lithography. Similar arrangements are used in other photolithographic applications, for instance for defining conductive patterns on a laminate which is a substrate for a large printed circuit board. In this case, located on stage 10, is a chuck (not shown) or other holding mechanism arrangement for holding the substrate (work piece). Other elements conventionally present on such a stage include fiducial marks and mechanisms for moving the chuck rotationally and up or down. These other elements also are not shown.
It is to be understood that also not shown are the remaining elements of the photolithographic system including an optical system for providing a light beam which is incident on the work piece located on stage 10, for imaging purposes. This optical system typically is located above the plane of the drawing. Also, the stage 10 typically moves on a large flat surface (not shown) and is supported thereon by air bearings or mechanical ball/roller bearings.
The interferometer system, to which the present invention is directed, in the prior art includes a one-piece large triangular (or rectangular or L-shaped) mirror 12, having two reflective surfaces M1 and M2. The single piece triangular or rectangular mirror 12 is carefully fabricated to subtend a 90 degree angle as shown between reflective surfaces M1 and M2. It is understood that typically the interferometry laser beams are in the visible wavelength spectrum and hence surfaces M1, M2 are reflective in the visible spectrum, but this is not limiting.
In this example, incident upon the so-called X direction surface M1, are two laser beams 16 and 18. In another embodiment only one laser beam is incident on surface M1. Use of two beams provides not only linear position data but also angular rotation data of the reflective surface. These beams 16, 18 are provided from interferometer mount 14. Typically there it is not an individual laser which provides each beam 16, 18 but instead a single laser coupled with a beam splitter provides the multiple beams. Note that for each laser beam, 16 or 18, there will be a corresponding reflected beam from the mirror M1 going back towards the interferometer mount, 14.
Also included in the interferometer mount 14 are optics and a high speed photodetector suitable for receiving the beams reflected back from mirror surface M1. These receiving optics, which are also conventional, generate the interferometer signal between each outgoing and received (reflected) interferometer laser beam and thereby determine the motion and thus the relative location in the X direction of reflective surface M1 by electronic analysis of the resulting interferometer signal and thereby the position of the attached stage 10 and of course the work piece.
A similar procedure is undertaken for determining the stage location in the Y direction using Y direction laser beams 26 and 28 incident on reflective surface M2 and reflected back to receiving optics at Y direction interferometer mount 24.
In order to maintain high precision, the orthogonality of the two reflective surfaces M1 and M2 must be nearly perfect. This is conventionally accomplished using a one piece solid triangular or rectangular or L-shaped mirror structure 12.
This arrangement works well for integrated circuit fabrication applications where a typical size of the wafer (work piece) is typically no more than 8 inches in diameter and the resulting length of each side of the stage 10 and hence mirror 12 is perhaps 12 inches. However, the present inventors have found that for applications where the stage 10 is intended to support a substantially larger substrate and hence the stage is scaled up substantially in size, the single piece mirror 12 becomes very large, hence too heavy and too expensive. Note that the expense of fabricating such mirrored surfaces increases (very approximately) with a cube of the length of the mirror. Also of course, the heavier the mirror, the more weight which the stage must support and hence more powerful motors are required to drive the stage, generating extra heat which must be removed and which degrades the accuracy of the positioning system due to thermal expansion of materials. Hence, a mirror 12 of the type shown in FIG. 1 is impractical for large substrates, i.e. 30 inch length or greater, of the type used in printed circuit board or flat panel display fabrication. It would be prohibitively expensive to fabricate a one piece mirror 12 as shown in FIG. 1 for a stage to support for instance a 60 inch length or longer substrate, which is the size likely to be used in the near future for flat panel displays. Hence there is a need for an interferometer mirror configuration which is less expensive and lighter and yet provides the needed precision for interferometry measurement.